Applicant has heretofore filed on Sep. 3, 1991, an invention entitled "Non-Toxic Corrosion Resistant Conversion Coating for Aluminum and Aluminum Alloys and the Process for Making the Same", Ser. No. 754,136.
As discussed in that earlier application, conversion coatings are employed on metals, notably aluminum and aluminum alloys whereby the metal surface reacts with a solution to convert to a corrosion protective film. Often, but not always, this protective film serves as a primer which may be top-coated with a paint for appearance purposes and also to enhance corrosion resistance. Heretofore, conversion coatings have employed chromates where maximum corrosion protection is desired or required. The most widely used chromate treatment for aluminum is the chromate-containing Alodine 1200 process (Alodine 1200 is manufactured and sold by Amchem Products, Inc., Ambler, Pa.). The Alodine process, however, puts chromates into waste water which are either not permitted or are severely restricted by the Environmental Protection Agency of the United States Government. Illustrative of such chromate uses in protective coatings are the U.S. Pat. Nos. 4,146,410 to Reinhold and 4,541,304 to Batiuk and the prior art references cited therein.
Where efforts have been made to avoid the use of chromates in conversion coatings special treatments are required which in most cases are either objectionable and unacceptable or do not provide the required or desired degree of corrosion resistance. Illustrative of such non-chromate coatings are the following U.S. Pat. Nos. 3,672,821 issued to Schlussler and 3,964,936 issued to Das. Also and more closely related to the present invention is the Great Britain patent 2 195 338A issued to Sanchem, Inc. and Paper No. 197 from CORROSION 86, entitled "Cationic Film Forming Inhibitors for the Protection of 7025 Aluminum Alloy Against Corrosion in Aqueous Chloride Solution" by Arnott, Hinton and Ryan presented at the annual meeting of the National Association of Corrosion Engineers, Mar. 17-21, 1986.
The Sanchem patent proposes a non-toxic conversion coating process employing relatively high alkaline solutions (pH 7 to 14) and is limited to in-house or laboratory use because of the elevated temperatures (at least 150.degree. F.) required. Moreover, the coating produced by Sanchem has limited corrosion inhibition, not acceptable in severe aqueous saline environments, notably MIL-C-5541C referred to above.
The Arnott et al. article recognizes the use of cerous chloride in lieu of a chromate to improve corrosion inhibition of aluminum. However, to be effective, exposure of the aluminum specimens to the cerous chloride is required for a prolonged time, on the order of 65 hours, which is unacceptable in production use. Moreover, the coated aluminum still fails to meet the corrosion protection requirements in severe aqueous saline environments.
Separately and apart from the foregoing, present day conversion coatings as illustrated by the above cited patents and publication, are readily wetted by moisture. It is well known that corrosion resistance of coatings is not as good as it could be if moisture were repelled, i.e., the coating were hydrophobic.
At the same time there is a problem in making surfaces hydrophobic. Paint topcoats will not adhere to surfaces which are highly hydrophobic, i.e., surfaces which have too low a surface energy. Surfaces readily wetted by water have energies greater than 65 dynes/cm. while surfaces such as polyethylene and teflon which have surface energies of approximately 25 dynes/cm. are not readily wetted by moisture or solvents. Consequently it is difficult to get adequate paint adherence on surfaces having low energy. However, it was demonstrated that the standard epoxy-polyamide paint (MIL-P-23377) used on Air Force and Navy aircraft will adhere well to surfaces having an energy at or above 40 dynes/cm.
In that earlier application a non-chromate chemical conversion coating process is described which involves a five minute treatment in a cerous chloride--potassium permanganate solution at room temperature and a subsequent 10 minute treatment in a hot sodium nitrite--sodium molybdate--sodium metasilicate solution.
Although this process produces a corrosion resistant coating on aluminum and aluminum alloys, it in some cases may not be economical for production. The cerous chloride--potassium permangante solution weakens over time and becomes unstable making it objectionable in cases where a lengthy shelf life is desired. A dark brown precipitate is gradually formed in the bath indicating undesirable chemical activity and a loss of effectiveness. Inasmuch as the cerous chloride is fairly expensive, this may make the process in some production situations too costly to operate. Also, the potassium permanganate solution is a deep purple color which might present waste disposal problems and objections from the Environmental Protection Agency of the U. S. Government.
The concentration of cerous chloride in the cerous chloride potassium permanganate used in the original process was 10 grams per 100 ml of water. The results of significant experiments have demonstrated this concentration can be reduced by a factor of 10. This greatly reduces not only the cost of operating and maintaining the bath but also increases the corrosion resistance of treated specimens. For example, a five minute immersion in a permanganate solution containing 10% cerous chloride produces 7075-T6 aluminum specimens with a corrosion rate of 2.02 mils/yr. Specimens treated in a solution containing only 1% cerous chloride have a corrosion rate of 0,165 mils/yr. The corrosion rates were determined by conducting potentiodynamic corrosion tests in aerated 0.35% NaCl solution. The improvement in corrosion resistance is probably due to the morphology of the mixture of cerous and aluminum oxides and hydroxides in the gold layer which is formed on the surface of the specimen. An added benefit is that the more dilute solution is more stable and will retain its activity longer than the solution which contains 10 % cerous chloride.
A second major improvement in the formulation was effected following a reduction in the cerous chloride concentration permanganate. When a 7075-T6 aluminum specimen was immersed in a solution containing 50 milliliters (ml) of deionized water, 0.5 grams of cerous chloride and 5 ml of 30% hydrogen peroxide for 5 minutes at room temperature a conversion coating was obtained and the corrosion rate of the specimen was 0.43 mils/yr. Although this was not as low as the 0.165 mils/yr produced by the permanganate solution, the performance in immersion tests in 3.5% NaCl was much better. The specimens treated with the permanganate solution began to turn black after 5 days but the specimens treated with the hydrogen peroxide solution retained the original gold color.
The hydrogen peroxide solution eliminates the waste disposal problem inherent in the permanganate solution and is also more stable. After one week the hydrogen peroxide solution was reactivated by adding more hydrogen peroxide. An aluminum specimen treated in the reactivated solution had a corrosion rate of 0.43 mils/yr, the same as specimens treated with a fresh solution.
For maximum corrosion resistance specimens treated in the improved solution would still have to be immersed in a hot (200 degree F.) molybdate--nitrite--silicate solution for ten minutes. However, the use of the improved solution minimizes the problem of bath stability and also significantly reduces the cost of the process.
Corrosion resistance is further improved by an added layer or overcoat produced by treating the coated aluminum surface with an alcoholic solution containing glycidoxy(epoxy)polyfunctionalmethoxysilane alone or in combination with phenyltrimethoxysilane. The particular alcohol used in these solutions are, for example, ethyl, isopropyl or methyl which are known to be equally effective as solvents for the silanes.
The above and other objects and advantages of the present invention will become more apparent from the following detailed description included in the best mode for carrying out the invention.